As integrated circuit (IC) process technology advances to higher densities, the feature size of a transistor is reduced, enabling low-voltage high speed operation and high density layout. Another result of the reduced feature size is to lower the transistor's gate oxide voltage tolerance. These transistors use lower supply voltages. For backward compatibility, a system can have different ICs having different supply. So one bus can be connected to different I/Os which are different supplies.
In many conventional input buffers, a weak pull up is used on input buffer to pull up the pad when it is not driven to avoid current flowing into the input buffer circuitry. In some conventional applications the pull up needs to be high voltage tolerant, without excess input high current (lih). In addition, the weak pull up must be turned off when the pad voltage is higher than the power supply (vpwr), and the input pull up circuit should not take any current.
A conventional pull-up input buffer solution is shown in FIG. 1. The conventional solution comprises a pad coupled to a pull up resistor, the other end of which is coupled to the drain of a transistor. The source of the transistor is coupled to power (Vpwr), and the gate of the transistor is couple to an enable signal. The pad is also coupled to the input of an input buffer.
Disadvantages of the conventional solution include that when the pull up is enabled, if the pad voltage is higher than the power supply (vpwr) then the pull up circuit will sink the current from the pad. Current will flow from the pad to vpwr through the transistor and the parasitic diode of transistor, which is undesirable.
In the conventional solution when the pull up option is disabled (enable signal (en_n) is connected to (vpwr), if the pad is higher than vpwr plus one threshold voltage (vpwr+VT) then the pull up circuit will still sink the current from pad (VT is the threshold voltage of PMOS transistor). Current will also flow from the pad to vpwr through the transistor and the parasitic diode of the transistor.
It would be desirable to have a high voltage tolerant pull up option.